1. Technical Field
The embodiments herein generally relate to orthogonal frequency division multiplexing (OFDM) communication systems, and, more particularly, to a method and system for Fast Fourier Transform (FFT) trigger point tracking for channels in OFDM based communication systems.
2. Description of the Related Art
OFDM systems are used for transmitting digital signals. In OFDM systems, data is provided with a number of orthogonal sub carriers, and are assigned to the amplitude and the phase of each sub carrier to perform digital modulation. The OFDM systems have wide applications in digital terrestrial broadcasting. Standards employing the OFDM system for terrestrial broadcasting include Digital Video Broadcasting-Terrestrial (DVB-T), Integrated Services Digital Broadcasting-Terrestrial (ISDB-T), and Integrated Services Digital Broadcasting-Digital Sound Broadcasting (ISBD-TSB) among others.
In the existing OFDM systems, signals are transmitted in the form of OFDM symbols. To achieve transmission in orthogonal sub channels, an OFDM symbol in the frequency domain is converted to the time domain by applying an Inverse Fast Fourier Transform (IFFT) procedure. An OFDM symbol includes a valid symbol and a guard interval. The valid symbol is a signal period when IFFT is performed during transmission. To assure that orthogonality is maintained in dispersive channels, the guard interval is added to the resulting time domain sequence; i.e., the valid symbol.
The guard interval is a copy of the waveform of a part of the second half of the valid symbol and may have a time length of ¼ or ⅛ of that of the valid symbol. The guard interval is appended in the first half of the OFDM symbol. The time duration for which the guard interval occupies the OFDM symbol is known as Tg (guard interval time). In existing OFDM systems, the guard interval should be at least as long as the duration (i.e., channel length) of the impulse response of the channel to ensure orthogonality. The impulse response of the channel (hereinafter referred to as “channel impulse response”) is the delay response of the channel while processing an incoming OFDM symbol in frequency domain during its transmission.
The duration of the channel impulse response (i.e., channel length) is the time duration until the end of reception of an incoming OFDM symbol. In the existing OFDM systems, the channel impulse response is calculated by a moving average filter. The moving average filter has a fixed moving average window of Tg. The peak position of the moving average output of an OFDM symbol is the desired trigger point for the OFDM symbol. The occurrence of a trigger point is used by a receiver apparatus of the existing OFDM systems to start the process of de-modulation of the OFDM symbol by applying a FFT.
Usually, in the existing OFDM systems, like ISDB-T, ISDB-TSB, and DVB T/H the FFT trigger point tracking is based on the channel impulse response, with the channel length (or delay spread) of the channel impulse response being smaller than the guard interval. FIG. 1 illustrates a conventional system 100 for trigger point tracking in an OFDM system. The conventional system 100 includes a time-domain interpolator 102 for interpolating incoming OFDM symbols in the time domain to form a time-domain interpolated channel. Further, an inverse FFT block 104 performs an inverse FFT on the time-domain interpolated channel to obtain a channel impulse response.
Thereafter, the channel impulse response is fed to a moving average filter 106. The moving average filter 106 has a fixed moving average window size Tg, where Tg is the guard interval time for an OFDM system. The moving average filter 106 uses the Tg to limit the time of processing of the OFDM symbols. A peak detection block 108 detects the peak position of the moving average output from the moving average (MA) filter 106. The peak position indicates the desired trigger point. However, in the existing OFDM systems, the trigger point may not be determined accurately for long channels. The long channels are prone to aliasing.
The aliasing causes the continuous signals in the long channels to become indistinguishable during processing by the moving average filter 106. Thereby, the channel impulse response detected by the moving average filter 106 for long channels may not represent the accurate channel impulse response. Since, the trigger point detection is based on the channel impulse response; an inaccurate channel impulse response may lead to errors in the detection of the trigger point for long channels. Additionally, the traditional design 100 may not detect the trigger point accurately in the long channels due to echo channels.
In some instances, echoes in long channels may occur outside the guard interval. In these instances the moving average filter 106 may process only the main path in the channel and may miss out on the echo channels. Since, the moving average filter 106 has a fixed moving average window size Tg, the moving average filter 106 may not detect the echo channels causing inferior trigger points and results in large inter-symbol interference.